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Due to rapidly changing physical and biochemical characteristics of growing leaves, correlations between traits of foliage biochemistry and the performance indices of flush feeding herbivores may vary considerably following relatively minor changes in experimental conditions. We examined the effects of the seasonal and inter-tree variation of a comprehensive array of biochemical compounds on the success of an early season geometrid, Epirrita autumnata, feeding on maturing foliage of mountain birch, Betula pubescens ssp. czerepanovii. We monitored the concentrations of individual phenolics, sugars, total nitrogen, nitrogen of proteins, and nitrogen of soluble compounds, water and acetone-insoluble residue. Simultaneously we recorded larval consumption, physiological performance, growth, and pupal mass of E. autumnata. We found significant phenological changes in almost all leaf traits measured. In bioassays with half-grown leaves, leaf gallotannin concentrations showed a nonlinear effect: in trees with high foliar gallotannin concentrations (over 10 mg g), physiological performance was strongly reduced by high gallotannin concentrations. In trees with lower gallotannin concentrations, on the other hand, larval growth was reduced by soluble proanthocyanidins, not gallotannins. Differences between high and low gallotannin trees largely depended on phenology, i.e., on the age of leaves. However, not all the differences in leaf traits between late (with high gallotannin concentrations at the time of the bioassay) and early flushing trees disappeared with leaf maturation, indicating that there is also phenology-independent variance in the tree population. In the full-grown leaves of all the study trees, low concentrations of water and of nitrogen of proteins (but not nitrogen of soluble compounds) were the main factors reducing pupal masses of E. autumnata, while neither gallotannin nor proanthocyanidins now played a significant role. The observed change in the factors underlying leaf quality (from gallotannins and proanthocyanidins to nitrogen and water) relate to the activity of the shikimate pathway and the formation of cell walls: gallotannins and proanthocyanidins are both produced in the pathway, and these tannins are assumed to contribute - via binding into cell walls - to tough and durable cell walls. Interestingly, low quality of leaves did not automatically translate into low foliar consumption (i.e., benefits to the tree). On the trees with young, high gallotannin leaves, larvae actually increased consumption on low quality foliage. In the group of trees with slightly more developed, low gallotannin leaves, the quality of leaves did not clearly modify amounts consumed. In full-grown leaves, low leaf quality strongly reduced leaf consumption. These results emphasize the strong influence of tree phenology on the relationships between biochemical compounds and the herbivore.

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We studied variation in leaf morphology and phonology in a population of the mountain birch (Betula pubescens ssp. tortuosa) in northernmost Finland over two summers. We tested tree quality with bioassays using several leaf-eating insect species to study the impact of plant variation on herbivore performance. Leaf size and phenology varied considerably between years. Differences among trees were significant, but individual trees retained their leaf-character-ranking within and between seasons. Positive correlations between tree height and leaf size and differences in bud growth patterns indicate that the mountain is genetically related to the dwarf birch, B. nana. On tall, large-leaved trees, which flushed early in spring, herbivores grew fast, whereas lower nana-like trees with smaller leaves leafing out late represented poor quality food. This pattern varied between years, being stronger in a summer when the trees produced relatively small poor quality leaves than in a summer when the trees produced larger leaves on which the herbivores grew faster. However, the mean growth rates of herbivores on individual trees were significantly correlated between the years. In general, tree quality seems to depend on the temperature conditions of the previous summer, although a current year's high temperatures may mould leaf quality by altering the maturation pattern of the leaves. Polymorphism within a mountain birch population may be partially maintained by conflicting selection pressures: periods with advantageous climatic conditions and low herbivore pressure should favour taller, large-leaved trees, whereas the episodes of high herbivore pressure should favour low, small-leaved trees.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. The consequences of hybridization between two related birch species, the mountain and the dwarf birch (Betula pubescens ssp. tortuosa and B. nana), on the performance of insect herbivores were studied in Finnish Lapland. Leaf quality of both arboretum-grown birches and birches of a natural stand was bioassayed with 14 insect species (one geometrid lepidopteran, one argid, one cimbicid, and 11 tenthredinid sawflies). All the herbivore species belonged to the local birch leafchewer guild. 2. There were considerable differences among the herbivore species in the relative suitability of the two parent birch species. Larvae of different species grew equally well on the pure mountain birches and on the hybrids, irrespective of the mother tree. In none of the cases were the hybrid birches superior as an insect diet. Dwarf birches were the least suitable for most of the insect species. A pronounced seasonal decline in leaf quality was observed in the mountain birches and in hybrids but not in the dwarf birches.4. All the insect species were collected from mountain birches, and they differed in their responses to dwarf birches. Three kinds of responses were observed: two species totally refused to feed on dwarf birch, three species were indifferent, and nine species had their slowest growth on the dwarf birch. 5. Two sawfly species, Arge fuscinervis and Nematus viridis, were reared in shortand long-term growth trials on leaves from arboretum and natural stand birches. Arboretum and natural birches did not differ as a diet for the larvae, and short-and long-term growth trials produced consistent results. 6. When fed dwarf birch foliage, individuals of A. fuscinervis had slower growth, lower survival and delayed adult emergence in the next season than individuals fed on mountain birch or hybrids. There were no such effects on N. viridis. 7. The results indicate that hybridization and introgression between northern birch species enhances variation in the leaf quality for insect herbivores. Accordingly, hybridization between host-plants may diversify solutions whereby herbivore species adapt to their hosts.

Two strains of a geometrid defoliator, Epirrita autumnata, were used in bioassays to test existence and relative efficacy of rapid, wound-induced foliage resistance in two provenances of the white birch. One birch and one moth strain originated in the outbreak range of the moth and another outside it. Both birch provenances responded to manual leaf damage by changes in foliage quality which significantly retarded growth of the insects, reducing their pupal weights and protracting larval periods. Leaves which were previously damaged were lower quality as Epirrita food than adjacent intact leaves. Both of them were lower quality than intact leaves without damaged leaves nearby. Because of variance between years in the efficacy of the response, and because of different transfer distances of the provenances to the common garden where the experiments were performed, we could not ascertain whether there is any overall difference in the efficacy of rapid inducible responses between the provenances. Both moth strains were affected by wound-induced deterioration in foliage quality. There were no differences in how the moth strains experienced inducible resistance in the two birch provenances. Moths achieved relatively higher pupal weights on the birch provenance matching their origin. Moths from the outbreak range completed their larval period in a shorter time and pupated in a smaller size and, due to dependence of fecundity on size, had a lower potential rate of increase than insects outside the outbreak range.